1. Field of the Invention
This invention pertains to the general field of magnetic resonance contrast agents used for medical diagnostic and therapeutic applications. In particular, the invention relates to the utilization of synthetic copolymeric compounds in novel hybrid configurations comprising metal ions in conjunction with nitroxide compounds.
2. Background of the Invention
Nuclear magnetic resonance imaging (generally referred to as NMR or MRI) is based, among other factors, on the detection of spatial variations in the T1 and T2 relaxation times in the tissues under observation. Therefore, contrast agents have been developed for imaging purposes to increase the natural relaxivity of tissues (i.e, to decrease T1 and T2) at the sites of interest.
High relaxivity is desirable for several reasons. Because the contrast effectiveness of contrast agents is proportional in large part to relaxivity, an agent with much greater relaxivity can be administered in much lower dosages, thus reducing the overall toxicity to which a patient is exposed during MRI. Furthermore, extremely high relaxivity agents make it possible to design targeted agents (such as antibodies) labeled with ultra-high relaxivity contrast agents.
The prior art describes many compounds containing paramagnetic metal ions (such as gadolinium and manganese, for example) utilized as contrast agents in various chemical formulations. Such agents' relaxivity has been found to be proportional to the unpaired electrons in the metal ions, as well as being affected by water exchange and electron and rotational correlation times. Because these metal ions tend to be toxic, they need to be chelated with a ligand in order to reduce the body tissue's ability to absorb them. Contrast agents are thus created, for example, by combining diethylenetriamine pentaacetic acid (DTPA) with gadolinium and ethylenediamine tetraacetic acid (EDTA) with manganese. Unfortunately, though, as a result of the metal ion's combination with a ligand, the relaxivity of the metal ion is decreased significantly, thus also reducing its effectiveness as a contrast agent. As shown in the examples of Table 1, at least one of the relaxation parameters of metal ions generally decreases substantially when the ions are chelated.
TABLE 1 ______________________________________ Relaxivity of Metal Ions versus Chelates Contrast Agent R1 R2 ______________________________________ FeCl.sub.3 0.94 .+-. 0.06 1.14 .+-. 0.12 Fe-DTPA 0.7 .gtoreq.0.7* MnCl.sub.2 8.73 .+-. 0.52 39.45 .+-. 0.52 Mn-DPDP 2.8 3.7 Mn-EDTA-MEA 3.29 + 0.14 5.76 .+-. 0.13 GdCl.sub.3 8.1 .gtoreq.8.1* Gd-DTPA 4.33 .+-. 0.15 5.19 .+-. 0.10 ______________________________________ *R2 is always greater than or equal to R1 (Measurements were taken using 0.5 Tesla and 20 MHz.)
It is known that the relaxivity of metal-ion chelates may be increased by attaching the chelates to macromolecules such as albumin or dextran, as shown by Gibby in U.S. Pat. No. 4,822,594 (1989). In theory, the polymeric macromolecules should increase the correlation time of the molecule resulting in a decrease of T1 and, possibly, of T2, which would enhance NMR imaging. In practice, though, such improvements in relaxivity are relatively modest.
Another class of well-known contrast agents consists of compounds that comprise nitroxide free radicals as the paramagnetic material used to improve the relaxivity for MRI purposes. Inasmuch as nitroxide free radicals are metabolically reduced and converted to diamagnetic material in the body, their concentration rapidly decreases below useful levels for relaxation enhancement. Thus, in order to produce the desired image enhancement, nitroxide-based agents have to be used at high concentrations that may be unacceptably toxic. Therefore, nitroxides are often combined with other functional molecules in an attempt to extend their effectiveness at concentrations that are tolerable for diagnostic and therapeutic applications.
For example, U.S. Pat. No. 4,863,717 to Keana (1989) describes a long-lasting nitroxide-bearing contrast agent that comprises a large molecule having surfaces covered with nitroxide free radicals and a liposome molecule that encapsulates an oxidant used to reoxidize the reduced nitroxide group back to its paramagnetic form. In U.S. Pat. No. 5,135,737 (1992), Keana also discloses the use of branched organic structures terminating with amine groups to which pharmacologically active groups, such as nitroxides and metal ions, can be chemically attached. These amplifier molecules are utilized to provide a stable contrast agent by combining a plurality of contrast-enhancing groups (such as nitroxides or paramagnetic metal ions) in the various branches of the molecule with a reactive moiety for attaching the molecule to target-specific biomolecules at the site of interest.
In U.S. Pat. Nos. 4,834,964 (1989) and 5,104,641 (1992), Rosen describes MRI image-enhancing agents based on charged, stable, organic nitroxides. Specific classes of nitroxide compounds are disclosed for scanning the spinal cord and organs associated with the cardiovascular system of a patient.
In our copending applications we disclose the use of copolymers containing paramagnetic metal ions dispersed along the copolymer chain, which, because of the size and spatial distribution of the resulting molecule, tends to increase the relaxivity and stability of the contrast agent. In a continuing effort to refine the use of oligomers and polymers as base molecules for introducing paramagnetic material in the human body, the present invention is related to a discovery that enables the design and synthesis of contrast agents with a yet much higher relaxivity than previously reported.